Code Division Multiple Access (CDMA) modulation and other spread spectrum techniques now find widespread application in wireless systems such as cellular mobile telephones, wireless local area networks and similar systems. In these systems a connection is provided between a central hub or base station and one or more mobile or remote subscriber units. The base station typically includes a specialized antenna for sending forward link radio signals to the mobile subscriber units and for receiving reverse link radio signals transmitted from the mobile units. Each mobile subscriber unit also contains its own antenna for the reception of the forward link signals and for transmission of reverse link signals. A typical mobile subscriber unit may for example, be a digital cellular telephone handset or a personal digital assistant having an incorporated cellular modem, or other wireless data device. In CDMA systems, multiple mobile subscriber units are typically transmitting and receiving signals on the same carrier frequency at the same time. Unique modulation codes distinguish the signals originating from or intended to be sent to individual subscriber units.
Other wireless access techniques also use spread spectrum for communications between a centralized unit and one or more remote or mobile units. These include the local area network standard promulgated by the Institute of the Electrical and Electronic Engineers (IEEE) 802.11 and the industry developed wireless Bluetooth standard.
The most common antenna used in a mobile subscriber unit is a monopole. A monopole antenna most often consists of a single wire or other elongated metallic element. A signal transmitted from such a monopole antenna is generally omnidirectional in nature. That is, the signal is sent with approximately the same signal power in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna, element, is likewise omnidirectional. A monopole antenna therefore cannot differentiate between signals originating from one direction versus a different signal originating from another direction. Although most monopole antennas do not produce significant radiation in the elevation plane, the expected antenna pattern in three dimensions is typically a donut-like toroidal shape, with the antenna element located at the center of the donut hole.
Unfortunately, CDMA communication systems are typically interference limited. That is, as more and more subscriber units become active within a particular area and share access to the same base station, interference increases among them, and thus so does the bit error rate they experience. To maintain system integrity in the face of increasing error rates, often the maximum data rate available to one or more users must be decreased, or the number of active units must be limited in order to clear the radio spectrum.
It is possible to eliminate excessive interference by using directive antenna at either the base station and/or the mobile units. Typically, a directive antenna beam pattern is achieved through the use of a phased array antenna at the base station. The phased array is electronically scanned or steered in a desired direction by controlling the phase angle of a signal input to each antenna element.
However, phased array antennas suffer decreased efficiency and gain as arrays become electrically small as compared to the wavelength of the radiated signals. When phased arrays are used or attempted to be used in conjunction with a hand-held portable subscriber unit, the antenna arrays spacing must be relatively small and therefore antenna performance is correspondingly compromised.